Administration of IL-2-Anti-IL2mAb Complex Post-Allogeneic HCT: a New Approach to Facilitate Rapid and Stable Hematopoietic Chimerism Following Reduced Intensity Conditioning and Experimental HCT

Engraftment and the induction of tolerance to donor antigens is a central aim of allogeneic hematopoietic cell transplantation (HCT). Currently, the administration of Treg cells is being examined as a new approach to support these processes. Based upon the reported ability of IL-2 anti-IL-2 mAb complex (IACX) to expand CD4⁺CD25⁺ FoxP3⁺ (Tregs) in vivo, we hypothesized that the administration of this complex to allogeneic HCT recipients may expand host Treg cells and inhibit host anti-donor responses, thereby effectively facilitating hematopoietic engraftment. To begin to address these questions, the effect of IACX treatment on the frequency of circulating Tregs was examined. Two days following the second injection of IACX in normal or reduced intensity conditioned (RIC) mice, the % of circulating Tregs in IACX treated mice was strikingly increased vs. PBS treated controls. Notably, the expression of Treg CD25 (MFI) was also significantly elevated (p<0.05) in IACX treated mice, consistent with the expansion and activation of Tregs following IACX treatment. Varying regimens of the complex were then administered to RIC MHC-matched allogeneic recipients. B6 mice (RIC, 5.5 Gy TBI) transplanted with 4 × 10⁶ BALB. B TCD-BM (day 0) were administered IACX (rIL-2/JES6-1) at days −5 and −3 (pre-HCT) or days +3 and +5 (post-HCT). In multiple independent studies, IACX treatment post-HCT consistently induced markedly superior expansion of host Treg populations at 1 wk post-HCT. Analysis of donor cells (Ly9.1⁺) 1 wk post-HCT clearly demonstrated that recipients treated with IACX post-HCT had markedly increased levels of circulating donor cells compared to the pre-HCT and PBS treated recipients (p<0.0001). To address the hypothesis that Treg expansion and activation following IACX treatment could inhibit host anti-donor CD8 T cell reactivity, host CD8⁺ T cell responses to the immunodominant donor epitope (H60) were analyzed by tetramer staining during the first 3 wks post-HCT. The frequency of these cells in recipients administered IACX post-HCT was greatly reduced compared to pre-HCT and PBS treated groups. Finally, to determine the effect of IACX treatment on engraftment, long-term chimerism was examined. Analysis for donor chimerism at 25 weeks post-HCT demonstrated: that only recipients administered IACX post-HCT were chimeric, containing high levels of donor cells vs. IACX pre-HCT and PBS controls, i.e. 36.1 ± 10.9% SE, 1.2 ± 0.785 and 0.78 ± 0.18, respectively. These findings demonstrate that infusion of IL-2 anti-IL-2mAb IACX in RIC allogeneic HCT can induce stable, long-term donor hematopoietic chimerism. Notably, the timing of IACX administration was found to be crucial with respect to conditioning and hematopoietic progenitor cell transplantation. At least part of the mechanism appears to involve the suppression of host anti-donor T cell responses, which presumably facilitates the initiation of early chimerism during the process of engraftment by donor cells. This approach may provide a new strategy to promote successful hematopoietic engraftment by manipulating endogenous Treg cells and thereby obviating the need for Treg infusions in certain transplant settings.